See also

Simply using Ghostscript

For example, on a MacIntosh with Mac OS 10.6.2, Scribus 1.3.6, Ghostscript 8.71, this ghostscript command will knock a 1.9GB PDF file generated by Scribus down to less than 300MB which is very helpful :

Put the command line in a batch file in the same folder as the Scribus generated PDF, and the batch will create the new file via double click.

Using a Python Script

A linux only program.
Note: Not all PDFs can be reduced, sometimes they are even larger!
Script creates a back up of the original (filename.pdf.orig) as well as the new file (filename.new.pdf) and you require pdftops and ps2pdf14. A 5MB file may take about 20 seconds, a 100 MB file may take minutes.